A method of directly dipping semiconductor devices in a fluid to cool them has been disclosed in, for example, U.S. Pat. No. 4,590,538. The foregoing conventional method provides an arrangement that logic modules, memory modules and a power supply module for several IP (Instruction Processor) boards, which are formed into a stack of printed circuit boards on each of which a multiplicity of semiconductor devices are mounted, are collectively dipped in a refrigerant in one refrigerant container, and the refrigerant fluid is circulated by a refrigerant circulation pump disposed outside the container so that the semiconductor devices are cooled.
In general, an apparatus for cooling semiconductors must have excellent cooling performance by utilizing the forced convection and boiling heat transfer to dip and cool very high heat generation density devices of about several 10 W/cm.sup.2. In this case, the flow of the refrigerant that passes through a pipe connecting the semiconductor devices and the refrigerant cooling device is a so-called gas-liquid two-phase flow composed of bubbles and liquid which flow simultaneously. The gas-liquid two-phase flow sometimes encounters pulsations occurring in its void ratio, which is the volume ratio of bubbles, and in pressure loss in the fluid passage thereof even in a steady state thereof. Therefore, the two-phase flow is liable to become unstable. If a pressure change takes place in a system of pipes followed by propagation of the change to surfaces of the semiconductor devices, saturation temperature of the refrigerant at the surfaces of the devices changes in accordance with the pressure. Therefore, the refrigerant is brought into a non-boiled state, resulting sometimes in that a predetermined cooling performance cannot be obtained. Therefore, it is necessary to maintain hydrodynamic stability of the system in terms of securing stability of operation of the functional system, such as a computer or its logic modules.
However, the foregoing conventional technology seems to have been established without taking into consideration the problem of the stability of the two-phase flow necessarily occurring in the case where a portion of the refrigerant liquid boils while forming bubbles.
Since the technology disclosed in U.S. Pat. No. 4,590,538 has the arrangement that the modules for several IPs are collectively dipped in one refrigerant container, the refrigerant must be completely removed from the container whenever the module or the device is changed. Therefore, the maintenance work cannot easily be performed. When the maintenance work, after the refrigerant has been removed, is performed at a user site contamination substances, such as dust, is likely to be mixed into the container during the maintenance work. As a result, there is a fear that the contamination substances adversely affect the semiconductor devices. The foregoing fact raises a critical problem for a ultra-high-density module which is mounted in the form of bare chips and/or on a thin film circuit board.
A cooling apparatus of a type for circulating refrigerant fluid to each of modules for cooling semiconductor devices has been disclosed in U.S. Pat. No. 3,586,101. In this conventional technology, natural circulation force generated due to density difference occurring in the refrigerant fluid is used as a main drive force for circulating the refrigerant.
However, the structure mainly utilizing the natural circulation force generated due to the density difference in the refrigerant fluid as disclosed in U.S. Pat. No. 3,586,101 encounters a limitation present in a flow rate of the refrigerant that can be circulated. Therefore, recently used devices of a very high heat generation density of about several 10 W/cm.sup.2 cannot be cooled in terms of practical effectivity. What is worse, the refrigerant is usually boiled in the case where the very high heat generation density device of the foregoing type is cooled. Therefore, the flow of the refrigerant is made to be a two-phase flow including bubbles and, accordingly, the flow is likely to be unstable. Accordingly, the hydrodynamic stability must be maintained to improve the stability of the operation of the system. However, the technology disclosed in U.S. Pat. No. 3,586,101 does not seem to take into consideration the foregoing problem about the stability of the two-phase flow.
The technology disclosed in U.S. Pat. No. 3,586,101 having the arrangement, where the refrigerant fluid is circulated through each semiconductor devices-cooling module, has no special consideration about method of the maintenance work. Therefore, a similar problem occurs to that experienced with the technology disclosed in U.S. Pat. No. 4,590,538.
Japanese Utility Model Examined Publication No. 3-7960 has disclosed a structure in which a refrigerant is forcibly circulated to be sprayed to the heat generating devices (semiconductor devices) in a semiconductor devices-cooling module so that higher cooling performance is realized. However, the structure disclosed in Japanese Utility Model Examined Publication No. 3-7960 has not disclosed an overall refrigerant circulation system. Further, no consideration has been taken in this Publication about the hydrodynamic stability of the cooling system.
Japanese Patent Application Laid-Open (Unexamined Publication) No. 4-113660 has disclosed an improved structure of semiconductor devices-cooling module having partition walls 7A between adjacent ones of a plurality of semiconductor devices on a substrate.